The Imposed Work of Breathing Is Less with the Laryngeal Mask Airway Compared with Endotracheal Tubes

Total work of breathing (WOB) includes physiologic work (due to elastic and resistive components of respiratory system in all spontaneously breathing patients) and imposed resistive work (due to the resistance of the endotracheal tube [ETT], circuits, ventilator, etc.) (1). Total work is measured by integrating the changes in intraesophageal pressure (indirect measurement of intrapleural pressure from a balloon catheter) and tidal volume (from a sensor at the airway opening). Data from these measurements and measurement of chest wall compliance are processed using the Campbell diagram to determine total work (1–4). Inspiratory-imposed resistive work of breathing (WOBI) is a major component of the total WOB for a tracheally intubated, spontaneously breathing patient (5). Additional resistive work imposed by the breathing apparatus should not be underestimated. In one study in adult patients, WOBI accounted for 80% of the total work under some conditions (5). The ETT is a significant resistor in the breathing apparatus (6–8). Increased WOBI adds to the WOB, which is already increased in diseases such as bronchopulmonary dysplasia (9). Increased WOB may precipitate respiratory muscle fatigue (respiratory muscles fail as force generators), which leads to hypoventilation, hypercapnia, and hypoxemia (10,11). A sensible approach, particularly for those patients predisposed to developing respiratory muscle fatigue, is to decrease the WOBI by using either low-flow–resistant equipment, pressure support ventilation, or both (10). In this study, we compared the WOBI in an age-appropriate laryngeal mask airway (LMA) with that in an age-appropriate ETT using a physiologic, sinusoidal inspiratory flow waveform to simulate spontaneous breathing. Methods An in vitro lung model was used to simulate a spontaneously breathing neonate and 3- and 5-yr-old children (12). To simulate spontaneous breathing, a ventilator set to deliver a sinusoidal inspiratory flow waveform was connected to one inflatable bellows compartment of a two-bellows compartment test lung (Model TTL; MI Instruments, Grand Rapids, MI). The compartment, inflated by the ventilator, mimics the actions of the respiratory muscles and is connected to a contralateral bellows compartment via a lifting bar. The contralateral compartment serves as a “lung” with air moving in and out. During spontaneous inhalation, the compartment, inflated by the ventilator, causes the lung compartment to be displaced by an equal volume. When the ventilator cycles off, the lifting bar disengages, allowing passive exhalation of the lung compartment (Figure 1). A sinusoidal flow waveform was selected because this is the natural physiologic waveform of a spontaneously breathing patient. Figure 1: In vitro model used to measure inspiratory imposed resistive work of breathing with various size laryngeal mask airways and endotracheal tubes. Either a laryngeal mask airway or an endotracheal tube was connected to the adapter, and measurements were obtained during simulated spontaneous breathing (see Methods). I = spontaneous inhalation, E = exhalation.LMAs ranging from 1.0 to 2.5 and ETTs ranging from 3.0 to 5.0 mm inner diameter were fitted to the lung model via a plastic adapter (Table 1). The LMAs were held in place on the lung model via a special, soft silicone adapter (Figure 1). The LMA was firmly secured to the soft adapter using rubber cement to maintain an occlusive seal, assuring no leakage in tidal volume (VT). Age-appropriate variables for VT and breathing frequency were used to simulate breathing (Table 2) (12,13). The ETT and LMA sizes were selected based on the appropriate simulated age. Three inspiratory flow rate demands for each age were simulated by changing the inhalation to exhalation time ratios (I:E) on the lung model, i.e., lowest flow demand (1:2), moderate flow demand (1:4), and highest flow demand (1:6). Table 1: Specification of Internal Diameter (ID) of a Laryngeal Mask Airway (LMA) Compared with an Endotracheal Tube (ETT)Table 2: Respiratory Variables Used to Simulate Age-Appropriate Spontaneous BreathingInspiratory flow rate demand and VT, measured using a miniature flow sensor (pneumotachograph) positioned between the lung model and the ETT or LMA, and the change in airway pressure measured distal to the ETT or LMA, were directed to a pediatric respiratory monitor (BEAR-BICORE, Riverside, CA), with a transducer pressure range of approximately −50 to 120 cm H2O (Figure 1). WOBI for the ETTs and LMAs were displayed in real time by integrating the changes in airway pressure within the inspiratory portion of the pressure-volume loop and VT (Figure 1). The aforementioned monitor has been validated as a means for accurately measuring imposed and physiologic WOB in pediatric and adult patients (12,14) and imposed WOB in laboratory and clinical studies (5,10,15). For each size LMA and ETT, five measurements were obtained at each flow rate demand. Data were analyzed by using a two-factor repeated-measures analysis of variance; α was set at <0.05 for statistical significance. Results WOBI was greater using age-appropriate ETTs compared with LMAs for all age groups. WOBI was significantly higher with the ETTs, particularly at the moderate to highest peak inspiratory flow rate demands (Figures 2 and 3). Under some conditions, WOBI was negligible using the LMA. At the highest peak inspiratory flow rate demand (1:6), using a 4.5-mm ETT, WOBI was as high as approximately 1.75 J/L. Figure 2: Simulated spontaneous breathing for a 5-yr-old child breathing through an age-appropriate laryngeal mask airway (LMA) and an age-appropriate endotracheal tube (ETT) at a moderate peak inspiratory flow rate demand (36 L/min) (inspiratory to expiratory ratio 1:4). The inspiratory imposed resistive work of breathing (WOBI ) is the area circumscribed within the inspiratory portion (shaded) of the pressure-volume loop, which moves in a clockwise direction during spontaneous inhalation (I) and exhalation (E). WOBI increased by approximately 910% with the ETT compared with the LMA, an excessive increase.Figure 3: Imposed work of breathing for a neonate and 3- and 5-yr-old children. P < 0.05 compared with a laryngeal mask airway (LMA) within each group at an inspiratory to expiratory (I:E) ratio of *1:2 (lowest spontaneous inspiratory flow rate demand), an I:E ratio of †1:4 (moderate inspiratory flow demand), and an I:E ratio of ‡1:6 (highest inspiratory flow rate demand). Data are mean ± SD.Discussion Unlike previous investigators (16,17), we measured WOBI during spontaneous ventilation using a sinusoidal inspiratory waveform, rather than a continuous flow waveform. This methodology more effectively reflects the normal breathing pattern. Using a model previously determined to measure WOBI accurately (12), our findings revealed significant increases in WOBI for all test conditions using pediatric age-appropriate ETTs compared with LMAs, particularly in neonates. In the neonatal simulation, WOBI was negligible using the LMA at all inspiratory flow rate demands. Increased WOBI increases respiratory muscle afterloading, which predisposes patients to fatigue, hypoventilation, and compromised gas exchange. If spontaneous breathing is desired, then decreasing the breathing apparatus resistance and WOBI by using low-flow–resistant equipment is a sensible approach for patients with decreased respiratory system compliance. Variations in WOBI between the LMAs and ETTs resulted from the interaction between the internal diameter of the tubes and the inspiratory flow rate demand. At the highest spontaneous inspiratory flow rate demand condition in our study (1:6), turbulent conditions no doubt existed, and the decrease in pressure from baseline was greatest. Thus, while maintaining a constant VT for each condition tested, the greater the decrease in pressure from baseline, the greater the measured imposed resistive WOB (Work = ∫ airway pressure dVT). WOBI increased significantly when VT was held constant, comparing the moderate with the highest flow rate demand condition, as for the 5-mm inner diameter ETT versus the 8.4-mm inner diameter LMA (size 2.5), for example (Figures 2 and 3). When flow rate demands were increased from the moderate to the highest level with the 5-mm ETT, WOBI increased by 60% (Figure 3). Comparing the size 2.5 LMA with the 5-mm ETT at the highest flow rate condition (54 L/min), WOBI increased by 700% (Figure 3). WOBI was negligible with an LMA under some conditions. Considering the appropriate pressure range and sensitivity of the respiratory monitor’s transducers, the data fall within expected limits. Under some test conditions, the flow rate and resistance to a given flow rate were probably so low that, although a VT change was recorded, no measurable pressure decrease across the device was detected. For example, using a size 1 LMA in the neonatal simulation at the 1:2 I:E ratio setting, a flow rate of approximately 3.5 L/min over 0.66 seconds was directed through a 5.25-mm tube. With this very low flow rate applied over a fleeting period, resistance to flow through the tube is negligible. Thus, the pressure decrease is essentially zero (change in pressure is proportional to resistance times flow rate). With no pressure change, although there is a volume change, no work is generated (Work = ∫ pressure dVT). Our study confirms the quantitative difference in WOBI when comparing an age-appropriate LMA with an age-appropriate ETT. These data can be extrapolated to the clinical situation, but the role of the glottis and its potential impact on airflow resistance with the LMA requires further investigation.